Decoding Device, Encoding Device, Decoding Method, Encoding Method, and Non-Transitory Computer Readable Recording Medium

ABSTRACT

A coding device and a decoding device are configured to include an estimated prediction mode deciding section ( 122   a ) including: a reference block selecting section ( 1223   a ) for selecting a reference block for use in estimating an intra-prediction mode for a subject block; an estimating number deciding section ( 1222   a ) for deciding an estimating number of estimated values to be estimated for the intra-prediction mode; and an estimated prediction mode deriving section ( 1221   a ) for estimating, based on a reference block, the estimating number of estimated values of the prediction mode.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. application Ser. No.16/262,862 filed Jan. 30, 2019, which is a Continuation of U.S.application Ser. No. 15/882,592 filed Jan. 29, 2018 (now U.S. Pat. No.10,230,963), which is a Continuation U.S. application Ser. No.15/357,833 filed Nov. 21, 2016 (now U.S. Pat. No. 9,924,173), which is aContinuation of U.S. application Ser. No. 15/162,282 filed May 23, 2016(now U.S. Pat. No. 9,538,198), which is a Continuation of U.S.application Ser. No. 13/810,081 filed Jan. 14, 2013 (now U.S. Pat. No.9,380,301), which is the National Phase of Patent Cooperation TreatyApplication No. PCT/JP2011/066055 filed on Jul. 14, 2011, which claimspriority to Japanese Patent Application No. 2010-160266 filed on Jul.15, 2010, the entire contents of which are hereby incorporated byreference.

TECHNICAL FIELD

The present invention relates to an image coding device for coding animage and an image decoding device for decoding a coded image.

BACKGROUND ART

In order to efficiently transmit or record a moving image, a movingimage coding device for coding a moving image and a moving imagedecoding device for decoding a moving image are widely used. Typicalexamples of a moving image coding method encompass a method adopted inH.264/MPEG-4.AVC (Non-patent Literature 1) and a method adopted in KTAsoftware which is a codec for joint development in VCEG (Video CodingExpert Group).

In these coding methods, each of pictures constituting a moving image ismanaged by a hierarchical structure that is constituted by (i) a slicewhich is obtained by dividing a picture, (ii) a macroblock which isobtained by dividing the slice, and (iii) a block which is obtained bydividing the macroblock.

The moving image coding device codes a moving image per block.Specifically, the moving image coding device performs (1) DCT (DiscreteCosine Transform) transformation of a prediction residual for eachblock, which prediction residual is obtained by subtracting a predictedimage from an original image, (2) quantization of a DCT coefficientwhich is obtained from the DCT transformation, and (3) a variable lengthcoding of the DCT coefficient thus quantized. In general, reduction ofan amount of codes is possible by performing, not for an original imagebut for a prediction residual, the DCT transformation, quantization, andvariable length coding.

The moving image coding device generates the above-mentioned predictedimage on the basis of a locally decoded image which is obtained byperforming inverse quantization and inverse DCT transformation of theDCT coefficient that has been quantized. As methods for generating thepredicted image, there are inter-screen prediction (inter prediction)and intra-screen prediction (intra prediction). In the moving imagecoding device, a prediction method to be used is selected for eachmacroblock. Note that because the present invention relates to intraprediction, the following discusses a case where the intra prediction isselected and used, and explanations on a process regarding the interprediction is omitted here.

Meanwhile, the moving image decoding device decodes a moving image perblock. Specifically, the moving image decoding device obtains a decodedimage, by performing (1) decoding of a DCT coefficient which has beensubjected to variable length coding and quantization, (2) inversequantization of the DCT coefficient that is obtained by the decoding,and (3) adds, to a predicted image, a prediction residual which isobtained by inverse DCT transformation per block. A method forgenerating the predicted image in the moving image decoding device isidentical with that in the moving image coding device.

The moving image coding device and the moving image decoding device ofthe conventional art each use a value that is called an intra-predictionmode (hereinafter, simply referred to as prediction mode), forgenerating an intra-prediction image per block. The prediction mode fora subject block has a value associated with one estimated value that isestimated from an adjacent block neighboring the subject block(Non-patent Literature 1). The estimated value is called an estimatedprediction mode. The moving image coding device codes the value of theprediction mode by use of the estimated prediction mode. Similarly, themoving image decoding device decodes, by use of the estimated predictionmode, the prediction mode from coded data which is generated by themoving image coding device. In a case where the estimated predictionmode is identical with an actual prediction mode, coding is performed insuch a way that a code length is shorter than that in a case where theactual prediction mode is directly coded.

In recent years, to cope with a higher definition of a digital imagetoday, coding by use of a larger macroblock has been attempted.Moreover, in order to perform more precise intra prediction for a largemacroblock, it has been attempt to increase a total number of predictionmodes by fractionalized prediction directions. (b) of FIG. 14 is a viewillustrating 32 types of prediction directions as a specific example ofmultiple directional predictions. For example, in a technique disclosedin Non-patent Literature 2, it is possible to employ a prediction modethat involves (i) more processing units (blocks) as compared toNon-patent Literature 1 and (ii) multiple directional predictions inmore directions which are similar to those shown in (b) of FIG. 14. Inparticular, for respective blocks dimensions of which are 16.times.16and 32.times.32, a total number M of the prediction modes=33.

CITATION LIST Non-Patent Literature 1

-   “Recommendation ITU-T H.264,” Telecommunication Standardization    Sector of ITU, March 2009 (Publication Date: March, 2009)

Non-Patent Literature 2

-   Ken McCann et. al., “Samsung's Response to the Call for Proposals on    Video Compression Technology (JCTVC-A124),” 1st Meeting: Dresden,    Del., 15-23 Apr. 2010 (Publication Date: April, 2010)

SUMMARY OF INVENTION Technical Problem

The inventor of the present invention found out that in a case where ablock for which a total number of prediction modes is increased isprocessed with only one estimated prediction mode, a possibility thatthe estimated prediction mode is identical with an actual predictionmode tend to decrease and consequently an efficiency in coding theprediction mode tends to decrease.

In order to prevent the efficiency in coding from decreasing, increasingthe number of the estimated prediction modes is conceivable to increasethe possibility that the estimated prediction mode is identical with theactual prediction mode. However, increase in total number of theestimated prediction modes leads to increase in amount of codes requiredfor coding the estimated prediction modes and the actual predictionmode. As such, the efficiency in coding may not be actually improved.

The present invention is accomplished in view of the above-mentionedproblem. An object of the present invention is to provide anintra-prediction mode estimation device which can realize such imageprediction mode estimation in a process of coding or decoding a digitalimage including a plurality of blocks that can (i) suppress an increasein coding required for coding an estimated prediction mode and aprediction mode and (ii) increase a possibility that the estimatedprediction mode is identical with an actual prediction mode at the sametime, thereby to increase an efficiency in coding of the digital image.

Solution to Problem

In order to solve the above problem, an intra-prediction mode estimationdevice of the present invention for performing intra-prediction modeestimation of an image, in a coding process or a decoding process for adigital image constituted of a plurality of blocks, includes: aselecting section for selecting one or more reference blocks forcomparison with a subject block, which is a subject for theintra-prediction mode estimation; an estimating number deciding sectionfor deciding a total number of estimated prediction modes to beestimated as estimated values in the intra-prediction mode estimationfor the subject block; and an estimating section for performing theintra-prediction mode estimation for the total number of the estimatedprediction modes with respect to the subject block, based on aprediction mode used for a coding process of the reference block thusselected.

In addition, in order to solve the above problem, a coding device of thepresent invention for coding a digital image constituted by a pluralityof blocks, includes: a selecting section for selecting one or morereference blocks for a subject block, which is a subject for the coding;an estimating number deciding section for deciding a total number ofestimated prediction modes to be estimated as estimated values inintra-prediction mode estimation for the subject block; an estimatingsection for performing the intra-prediction mode estimation for thetotal number of the estimated prediction modes with respect to thesubject block, based on a prediction mode used for a coding process ofthe reference block thus selected; a prediction mode deciding sectionfor deciding a prediction mode for the subject block; a prediction modecoding section for coding the estimated prediction modes and theprediction mode in such a way that a code length of the coding isdifferent according to whether or not the prediction mode is identicalwith any one of the estimated prediction modes; and a prediction imagegenerating section for generating a predicted image of the subjectblock, based on the prediction mode, the prediction mode coding sectionperforming the coding in such a way that the code length of the codingis shorter when the prediction mode is identical with any one of theestimated prediction modes than when the prediction mode is notidentical with any one of the estimated prediction modes.

Moreover, in order to solve the above problem, a decoding device of thepresent invention for decoding a coded data of a digital imageconstituted by a plurality of blocks, includes: a selecting section forselecting one or more reference blocks for a subject block, which is asubject for the decoding; an estimating number deciding section fordeciding a total number of estimated prediction modes to be estimated asestimated values in intra-prediction mode estimation for the subjectblock; an estimating section for performing the intra-prediction modeestimation for the total number of the estimated prediction modes withrespect to the subject block, based on a prediction mode used for acoding process of the reference block thus selected; a prediction modedecoding section for decoding, from the coded data, a prediction mode tobe used in generation of predicted image of the subject block, theprediction mode decoding section performing the decoding based on thetotal number of the estimated prediction modes and the estimatedprediction modes; and a prediction image generating section forgenerating a predicted image of the subject block, based on theprediction mode.

Furthermore, in order to solve the above problem, coded data of animage, which coded data, constituted by a plurality of blocks, isoutputted or is to be outputted from a coding device of the presentinvention, the coded data includes: a) information indicating that afirst estimated prediction mode for the block is identical with aprediction mode for the block; b) information indicating that a secondestimated prediction mode but not the first estimated prediction mode isidentical with the prediction mode; or c) (i) information indicatingthat the first estimated prediction mode and the second estimatedprediction mode are not identical with the prediction mode, and (ii)information regarding the prediction mode.

According to the configuration described above, it is possible toadaptively decide, for each subject block, a total number of estimatedprediction modes to be estimated. In general, use of a large number ofestimated prediction modes can increase a possibility that the estimatedprediction mode is identical with an actual prediction mode. However, ina case where none of the estimated prediction modes is identical withthe actual prediction mode, an amount of codes is increased. Accordingto the configuration described above, the total number of the estimatedprediction modes to be estimated is adaptively decided. This makes itpossible to efficiently code and decode a prediction mode by use of asmaller average amount of codes.

Advantageous Effects of Invention

As described above, the intra-prediction mode estimation device of thepresent invention for performing intra-prediction mode estimation of animage, in a coding process or a decoding process for a digital imageconstituted of a plurality of blocks, includes: a selecting section forselecting one or more reference blocks for comparison with a subjectblock, which is a subject for the intra-prediction mode estimation; anestimating number deciding section for deciding a total number ofestimated prediction modes to be estimated as estimated values in theintra-prediction mode estimation for the subject block; and anestimating section for performing the intra-prediction mode estimationfor the total number of the estimated prediction modes with respect tothe subject block, based on a prediction mode used for a coding processof the reference block thus selected.

Accordingly, it is possible to efficiently code and decode a predictionmode by use of a smaller average amount of codes by adaptively decidingthe total number of the estimated prediction modes to be estimated.

Further, as described above, in the moving image coding device of thepresent invention, even in a case where any of a large number ofprediction modes can be selected, efficient coding of a prediction modeis possible by using a smaller average amount of codes by selectivelyusing a plurality of estimated prediction modes. Moreover, the movingimage decoding device of the present invention can decode coded datawhich has been coded by such a moving image coding device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating a configuration of a predictioninformation coding section 12 a.

FIG. 2 is a view illustrating a configuration of an MB coding section 10a.

FIG. 3 is a view illustrating components of an estimated prediction modedeciding section 122 a.

FIG. 4 is a view illustrating a procedure for deriving an estimatedprediction mode.

FIG. 5 is a view illustrating a specific example of a predeterminedtable for deriving a degree of contribution.

FIG. 6 is a view illustrating a specific example of a state of a subjectblock and adjacent blocks neighboring the subject block. (a) of FIG. 6illustrates a case where a dimension of the subject block is identicalto a dimension of each of the adjacent blocks. (b) and (c) of FIG. 6illustrate a case where a dimension of the subject block is differentfrom a dimension of an adjacent block on the left side of the subjectblock.

FIG. 7 is a table showing a specific example of a coding method fortransforming a prediction mode to coded data.

FIG. 8 is a view illustrating a configuration of an MB decoding section20 a.

FIG. 9 is a view illustrating a configuration of a predictioninformation decoding section 23 a.

FIG. 10 is a view illustrating a specific example of a method fordecoding coded data to a prediction mode.

FIG. 11 is a view schematically illustrating a slice constituted bymacroblocks MB1 to MBn.

FIG. 12 is a view illustrating an MB coding section 10 in a conventionalmoving image coding device 1.

FIG. 13 is a view illustrating an MB decoding section 20 in aconventional moving image coding device 2.

FIG. 14 is a view illustrating a specific example of a prediction modewhich is associated with directions. (a) of FIG. 14 illustrates eighttypes of prediction directions, and (b) illustrates 32 types ofprediction directions.

FIG. 15 is a view illustrating a configuration of a predictioninformation coding section 12.

FIG. 16 is a chart illustrating a process flow in the predictioninformation coding section 12.

FIG. 17 is a view illustrating a configuration of an estimatedprediction mode deciding section 122.

FIG. 18 is a view illustrating a process in an estimated prediction modederiving section 1221.

FIG. 19 is a table illustrating a specific example of a coding methodfor transforming a prediction mode to coded data.

FIG. 20 is a view illustrating a configuration of a predictioninformation decoding section 23.

FIG. 21 is a chart illustrating a process flow in the predictioninformation decoding section 23.

FIG. 22 is a table illustrating a specific example of a method fordecoding coded data to a prediction mode.

DESCRIPTION OF EMBODIMENTS

The following description will discuss embodiments of the presentinvention, with reference to drawings. Note that components mentioned inpreceding description are respectively given the identical referencesigns to those assigned to components when the component is mentionedfor the first time is assigned to the component, and a description willnot be provided.

First, details of a coding process and a decoding process of theconventional intra prediction will be described below with reference tothe figures.

FIG. 11 is a schematic view illustrating a slice constituted bymacroblocks MB 1 to MB n. Further, FIG. 12 is a view illustrating an MBcoding section 10 in a conventional moving image coding device 1, andFIG. 13 is a view illustrating an MB decoding section 20 in aconventional moving image coding device 2.

A coding process in the moving image coding device 1 and a decodingprocess in the moving image decoding device 2 are performed as describedbelow in brief. First, the moving image coding device 1 performs thecoding process, with respect to a macroblock MB i (i=1 to n) which isincluded in a slice, in a predetermined order per slice (e.g., in orderof raster scan) (see FIG. 11). The coding process is performed by the MBcoding section 10, so that MB coded data is outputted.

On the other hand, the moving image decoding device 2 performs thedecoding process of the MB coded data, with respect to a macroblock MB i(i=1 to n) which is included in the slice. The moving image decodingdevice 2 performs the decoding process in a predetermined order perslice (e.g., in order of raster scan). The decoding process is performedby the MB decoding section 20, as a result of which an MB decoded imageis outputted. All of MB decoded images of the slice constituting apicture are outputted, so that one picture is completed.

In the moving image coding device 1 and the moving image decoding device2, each of macroblocks is processed processing units into which themacroblock is divided, the processing units being not greater than eachmacroblock in dimension. This processing unit is hereinafter referred toas block. Note that in order to simplify the description, the followingdescription is based on a case where a shape of the block is square.

Here, an internal process of the MB coding section 10 in the movingimage coding device 1 will be described with reference to FIG. 12. Asshown in FIG. 12, the MB coding section 10 of the moving image codingdevice 1 includes a prediction information deciding section 11, aprediction information coding section 12, a prediction image generatingsection 13, a prediction residual coding section 14, and an MB codeddata multiplexing section 15.

First, the prediction information deciding section 11 decides anintra-prediction mode and a block size. A prediction mode indicates amethod for generating the intra-prediction image, and the block sizeindicates a dimension of a block. Any method can be used to decide theintra-prediction mode and the block size. In general, a combination ofvalues which minimize an RD cost which is derived in consideration of acoding amount and a residual is adopted.

The prediction information coding section 12 generates predictioninformation coded data for the prediction mode and the block size.

The predicted image generating section 13 performs intra prediction inthe prediction mode and the block size, and outputs a predicted imagebased on the intra prediction. A sum of the predicted image and aprediction residual is a local decoded image of a subject block.

The prediction residual coding section 14 subjects a prediction residualto a DCT (Discrete Cosine Transform), quantizes the prediction residualobtained by the DCT, and outputs prediction residual coded data.

The MB coded data multiplexing section 15 multiplexes predictionresidual coded data with prediction information coded data, and outputsthus multiplexed data as MB coded data.

Here, the following discusses internal processing of a MB decodingsection 20 of the moving image decoding device 2. As shown in FIG. 13,the MB decoding section 20 of the moving image decoding device 2 isprovided with a MB coded data inverse multiplexing section 21, aprediction residual decoding section 22, a prediction informationdecoding section 23, and the prediction image generating section 13.

First, the MB coded data inverse multiplexing section 21 separatesvarious types of coded data from a single piece of coded data by inversemultiplexing. The MB coded data inverse multiplexing section 21 furtheroutputs prediction residual coded data to the prediction residualdecoding section 22, and outputs prediction information coded data tothe prediction information decoding section 23.

Next, the following discusses operation of another module inside of theMB decoding section 20.

The prediction residual decoding section 22 decodes prediction residualcoded data so as to obtain a DCT coefficient, and outputs predictionresidual by an inverse quantization/inverse DCT transformation of theDCT coefficient.

The prediction information decoding section 23 derives a prediction modeand a block size from a prediction information coded data, and outputsthe prediction mode and the block size thus derived.

The prediction image generating section 13 is identical with that of themoving image coding device 1. A predicted image generated by thepredicted image generating section 13 is added with a predicted residualand thereby becomes a MB decoded image.

Each internal processing mentioned above is performed with all blockscontained in a subject macro block in a same order of blocks as that ofthe moving image coding device 1. A set of local decoded images of allblocks becomes the MB decoded image in the subject macro block.

Here, coding of a prediction mode and operation of decoding in theprediction information coding section 12 and the prediction informationdecoding section 23 are described with reference to FIGS. 14 through 22.First of all, the following explains what are the prediction mode and anestimated prediction mode.

The prediction mode is a parameter indicating which method forpredicting is to be employed out of a plurality of methods forpredicting that can be selected discretionary in intra prediction. Thenumber of methods for predicting which can be selected as the predictionmode, namely, a total number M of prediction modes is decided accordingto, for example, a block size. Actual examples are, according totechnique of Non-patent Literature 1, M=9 for block sizes of 4.times.4and 8.times.8, and M=4 for a block size of 16.times.16. Generally, aprediction mode has two types of modes (i) for generating a predictedimage by causing all pixel values to be a same value, for example, anaverage value of adjacent pixels and (ii) for generating a predictedimage by extrapolating or interpolating each referred pixel value in apredetermined direction. FIG. 14 illustrates an actual example of aprediction mode associated with orientation. (a) of FIG. 14 is a viewillustrating a case where there are 8 types in the prediction mode.

Basically, a code amount according to the total number M of predictionmodes is necessary for coding of the prediction mode. For example, in acase of M=8, a code of 3 bits is simply required for selecting one of 8prediction modes. However, because a spatial correlation between pixelvalues is high, correlation between values of prediction modes is alsolocally high. In other words, it is possible to perform an estimation ofa prediction mode. It is a mechanism of an estimated prediction modethat employs the estimation of a prediction mode.

An estimated prediction mode is a prediction mode that is very likely tobe selected for a certain subject block. If the estimated predictionmode of the moving image coding device 1 is identical with that of themoving image decoding device 2, it is possible to perform derivation ofthe estimated prediction mode by any method. For example, in Non-patentLiterature 1, an estimated prediction mode for a subject block is aprediction mode having a mode value which is smaller than those ofprediction modes of blocks adjacent to the subject block in an upwarddirection and a leftward direction.

Next, a configuration and a workflow of the prediction informationcoding section 12 are described below with reference to FIGS. 15 and 17.FIG. 15 is a block diagram illustrating a configuration of theprediction information coding section 12. FIG. 16 illustrates a workflowof the prediction information coding section 12. As illustrated in FIG.16, the prediction information coding section 12 performs:

(Step S1) firstly, deriving an estimated prediction mode,

(Step S2) secondly coding a prediction mode by use of the estimatedprediction mode, and

(Step 3) recoding the prediction mode as reference for a later process.The following discusses members composing the prediction informationcoding section 12 which performs these processes, with reference to FIG.15. As illustrated in FIG. 15, the prediction information coding section12 is provided with a prediction mode recoding section 121, an estimatedprediction mode deciding section 122, and a prediction mode codingsection 123.

The prediction mode recoding section 121 stores therein information ofthe prediction modes for the respective macro blocks which have beenprocessed, and outputs the information when it is necessary to refer tothe prediction mode.

The estimated prediction mode deciding section 122 outputs the estimatedprediction mode on the basis of prediction modes of blocks adjacent tothe subject block. Here, a detailed configuration and a workflow of theestimated prediction mode deciding section 122 can be described belowwith reference to FIGS. 17 and 18. FIG. 17 is a view illustrating aconfiguration of the estimated prediction mode deciding section 122.FIG. 18 is a view illustrating a process performed in an estimatedprediction mode deriving section 1221. As illustrated in FIG. 17, theestimated prediction mode deciding section 122 is provided with theestimated prediction mode deriving section 1221. As illustrated in FIG.18, the estimated prediction mode deriving section 1221 performs:

(Step S4) acquiring, as candidates of estimated prediction modes,prediction modes employed in blocks adjacent to the subject block in anupward direction and a leftward direction, and

(Step S5) selecting, as an estimated prediction mode, an estimatedprediction mode having a mode value which is smaller than those of othercandidates of estimated prediction modes.

The prediction mode coding section 123 codes the prediction mode intoprediction information coded data on the basis of a block size and anestimated prediction mode for the subject block, and outputs theprediction information coded data. For example, the block size is codedas side information corresponding to a method for dividing a macroblock. A method for coding a prediction mode is described with referenceto FIG. 19.

FIG. 19 is a table illustrating an actual example of a method for codinga prediction mode of the prediction mode coding section 123. In FIG. 19,“m” stands for a value of a prediction mode, “p” stands for a value ofan estimated prediction mode, and “k=binary(x,B.sub.M′)” stands for aconversion of a value x into a B.sub.M′-digit bit column k. “B.sub.M′”is the number of bits required for showing a total number M′ ofprediction modes (which is equal to M−1 here) that is obtained byeliminating estimated prediction modes from a total number M ofprediction modes, and also is a number which can be obtained by aformula “B.sub.M′=ceil (log (M′))”.

As illustrated in FIG. 19, the prediction mode coding section 123outputs “flag_bits” and further outputs “rem_bits” under conditions thatare fulfilled by the prediction mode. The followings are three patternsof the conditions, and operations of the respective cases. (a) In a casewhere a prediction mode m is equal to an estimated prediction mode p,the prediction mode coding section 123 outputs only 1 bit of“flag_bit=1” as a coded data. (b) In a case where a prediction mode m isnot equal to an estimated prediction mode p, the prediction mode codingsection 123 outputs “flag_bit=0” and further outputs a bit column“rem_bits” which was derived, according to a value of m, by use of anyone of the following methods.

In case of (b1) m<p, the prediction mode coding section 123 converts minto a bit column “rem_bits”.

In case of (b2) m>p, the prediction mode coding section 123 converts m−1into a bit column “rem_bits”.

The description (b) is supplemented below. First, in a case of m>p, itis obviously not m=p in a case of (b). Therefore, in a case of m>p, itis advisable to displace m by m−1 and convert m−1 into a bit column sothat one range of value of a “rem_bits” can be reduced enabling it toprove to be useful in reduction in code amount.

As mentioned above, conventionally, a code amount of coded data of aprediction mode used to be reduced by use of one estimated predictionmode.

Next, a configuration and a workflow of the prediction informationdecoding section 23 are described below with reference to FIGS. 20 and21. FIG. 20 is a view illustrating a configuration of the predictioninformation decoding section 23. The prediction mode recoding section121 and the estimated prediction mode deciding section 122 illustratedin FIG. 20 are identical with components of the prediction informationcoding section 12. FIG. 21 is a view illustrating a workflow of decodinga prediction information coded data in the prediction informationdecoding section 23. As shown in FIG. 21, the prediction informationdecoding section 23 performs:

(Step S6) firstly, deriving an estimated prediction mode by theestimated prediction mode deciding section 122,

(Step S7) next, decoding a prediction mode by a prediction mode decodingsection 231 in accordance with a flag decoded from the predictioninformation coded data, and

(Step S8) further, recoding the prediction mode in the prediction moderecoding section 121 as reference for a later process.

The prediction mode decoding section 231 which decodes the predictionmode in Step S7 is described below with reference to FIG. 22.

FIG. 22 is a table illustrating an actual example of a method fordecoding coded data into a prediction mode in the prediction modedecoding section 231. In FIG. 22, k represents a B.sub.M′-digit bitcolumn. m′=decimal(k,B.sub.M′) represents a conversion of theB.sub.M′-digit bit column k into an ordinary scale m′. Note thatdescription of reference numerals identical to those of FIG. 19 isomitted here. A total number M of prediction modes in the subject blockwhich is required for deriving the number of digits B.sub.M′ can bederived in accordance with the block size obtained by decoding theprediction information coded data.

The prediction mode decoding section 231 first decodes “flag_bit”. Asshown in FIG. 22, when “flag_bit=1”, the estimated prediction mode p isequal to the prediction mode m. In this case, it is not required todecode a “rem_bits”, instead, the prediction mode m=p is outputted. When“flag_bit=0”, the prediction mode decoding section 231 converts, as the“rem_bits”, the following B.sub.M′-digit bit coded data k into anordinary scale m′ and derives the prediction mode m according tomagnitude relation between m′ and p shown in the lowest row of a tableof FIG. 22.

Accordingly, the conventional moving image coding devices 1 and 2 usedto code/decode a prediction mode by use of one estimated predictionmode.

Moving Image Coding Device

The following describes a moving image coding device 1 a of anembodiment of the present invention with reference to FIGS. 1 through 7.The moving image coding device 1 a of Embodiment 1 of the presentinvention is schematically identical to the conventional moving imagecoding device 1 in that each micro block of an input image is coded andthe input image thus coded is outputted as coded data. The moving imagecoding device 1 a is a moving image coding device which partiallyemploys a technique which is adopted in H264/AVC standard. FIG. 2 is aview illustrating a configuration of an MB coding section 10 a includedin the moving image coding device 1 a. The MB coding section 10 aillustrated in FIG. 2 includes a prediction information coding section12 a instead of the prediction information coding section 12 which isincluded in the MD coding section 10 illustrated in FIG. 12. Theprediction information coding section 12 a is identical to theprediction information coding section 12 in that prediction modeinformation and a block size are coded and prediction information codeddata is outputted. The following describes what is different between theprediction information coding section 12 a and the predictioninformation coding section 12.

FIG. 1 is a view illustrating a configuration of the predictioninformation coding section 12 a. The prediction information codingsection 12 a includes an estimated prediction mode deciding section 122a, a prediction mode coding section 123 a, and a prediction moderecording section 121 which is identical to that of the moving imagecoding device 1. The following specifically describes the estimatedprediction mode deciding section 122 a and the prediction mode codingsection 123 a in this order.

First, the estimated prediction mode deciding section 122 a is describedas follows with reference to FIG. 1. The estimated prediction modedeciding section 122 a illustrated in FIG. 1 is different from theestimated prediction mode deciding section 122 illustrated in FIG. 15 inthat the estimated prediction mode deciding section 122 a derives aplurality of estimated prediction modes. Derived estimated predictionmodes, and the number of the derived estimated prediction modes, thatis, an estimating number P are outputted as estimated prediction modeinformation. That is, the estimated prediction mode information isinformation which contains the estimating number P, and the estimatedprediction modes which are equal in number to the estimating number P.The following explains the estimated prediction mode deciding section122 a based on an exemplary case where the estimating number P is 2.However, the estimating number is not limited to 2 but may be 1 or notless than 3. Note that use of too many estimated prediction modes is notpreferable. This is because code amount is required so as to ultimatelyselect one estimated prediction mode from the too many estimatedprediction modes.

The following specifically describes a process to be carried out by theestimated prediction mode deciding section 122 a, with reference to FIG.3. FIG. 3 is a view illustrating components of the estimated predictionmode deciding section 122 a.

The estimated prediction mode deciding section 122 a obtainscontribution degrees of respective prediction modes of adjacent blocksadjacent to a subject block, compares the contribution degrees, andderives an estimated prediction mode. A contribution degree indicateshow probably a prediction mode for an adjacent block adjacent to asubject block is identical to that of the subject block. As illustratedin FIG. 3, the estimated prediction mode deciding section 122 a includesan estimating number deciding section 1222 a and a reference blockselecting section 1223 a.

The estimating number deciding section 1222 a decides the estimatingnumber P. The reference block selecting section 1223 a selects blockswhich are used so as to obtain respective estimated prediction modecandidates. An estimated prediction mode deriving section 1221 aderives, from the estimated prediction mode candidates, an estimatedprediction mode(s) which is(are) equal in number to the estimatingnumber P. The following describes how the estimated prediction modedeciding section 122 a derives an estimated prediction mode, withreference to FIG. 4. FIG. 4 is a view showing how the estimatedprediction mode deciding section 122 a derives an estimated predictionmode.

Step S1 a

The estimating number deciding section 1222 a decides an estimatingnumber P on the basis of a total number M of prediction modes of asubject block. Specifically, assume that a predetermined threshold Mtfor the total number M of prediction modes is 10. Under the assumption,the estimating number deciding section of Embodiment 1 decides that theestimating number P is 2 in a case where the total number M ofprediction modes is not less than the predetermined threshold Mt, anddecides that the estimating number P is 1 in a case where the totalnumber M of prediction modes is smaller than the predetermined thresholdMt. As early described, generally, probability that an estimatedprediction mode is identical to an actual prediction mode decreases asthe total number M of prediction modes increases. Therefore, theprobability that an estimated prediction mode is identical to an actualprediction mode can be increased by setting such that the estimatingnumber P becomes relatively large in a block having a large total numberM of prediction modes.

The total number M of prediction modes is a value decided on the basisof a block size of the subject block. For example, in a case where theblock size is 4.times.4 or 8.times.8, the total number M of predictionmodes is set to 9. In a case where the block size is 16.times.16 or32.times.32, the total number M of prediction modes is set to 33. In acase where the block size is other than 4.times.4, 8.times.8,16.times.16, and 32.times.32, the total number M of prediction modes isset to 5. It is therefore possible to derive the estimating number P onthe basis of the block size instead of the total number M of predictionmodes.

Step S2 a

The reference block selecting section 1223 a selects, as referenceblocks which are used so as to obtain estimated prediction modecandidates, three of (i) a block adjacent to a left side of the subjectblock (a leftward adjacent block), (ii) a block adjacent to an upperside of the subject block (an upward adjacent block), and (iii) a blockadjacent to an upper-left corner of the subject block (an left-upwardadjacent block). Note that in a case where a plurality of blocks areadjacent to a side of a subject block, for example, a left side of thesubject block, the top one of the plurality of blocks is selected as areference block. In a case where a plurality of blocks are adjacent toan upper side of the subject block, the leftmost one of the plurality ofblocks is selected as a reference block. The leftward adjacent block,the upward adjacent block, and the left-upward adjacent block arehereinafter referred to as a block L, a block U, and a block UL,respectively (see, for example, (a) of FIG. 6). In a case where a blockcannot be selected as a reference block such as a case where a subjectblock is located at an edge of a screen or a case where a block to beselected has not been processed, the block may not selected or anotherblock may be selected. For example, a block to be first processed amongblocks included in a frame includes no block which can be selected as areference block in the same frame. In the case, however, a virtual blockhaving a predetermined default may be selected as the reference blockfor the block to be first processed.

Step S3 a

The estimated prediction mode deriving section 1221 a derives estimatedprediction mode candidates and contribution degrees of the respectiveestimated prediction mode candidates on the basis of the referenceblocks thus selected. The estimated prediction mode candidates and thecontribution degrees are derived according to the following procedures.First, the estimated prediction mode deriving section 1221 a obtains aprediction mode mL of the block L, a prediction mode mU of the block U,and a prediction mode mUL of the block UL as the estimated predictionmode candidates.

Next, the estimated prediction mode deriving section 1221 a derives acontribution degrees cL of the estimated prediction mode candidate mL, acontribution degree cU of the estimated prediction mode candidate mU,and a contribution degree cUL of the estimated prediction mode candidatemUL, each of the contribution degrees cL, cU, and cUL being representedby a value of 0.0 through 1.0. The following describes, with referenceto FIGS. 5 and 6, (i) a table which is used for derivation ofcontribution degrees and (ii) the procedures for deriving thecontribution degrees. FIG. 5 exemplifies a predetermined table which isused for derivation of the contribution degrees cL, cU, and cUL of therespective prediction modes mL, mU, and mUL, in which table the totalnumber M of prediction modes is 9. Specifically, the predetermined tableshows contribution degrees of respective estimated prediction modecandidates (modes 0 through 8) for each of the leftward adjacent blockL, the upward adjacent block U, and the upper-leftward adjacent blockUL. According to the table illustrated in FIG. 5, for example, thecontribution degree of the estimated prediction mode candidate mL=1(mode 1) for the leftward adjacent block L is “1.0”. Note here that thenine prediction modes consist of eight directions (modes 0 through 7)illustrated in (a) of FIG. 14 and one planar prediction (mode 8).

How to derive the contribution degrees is described with reference toFIG. 6. FIG. 6 is a view illustrating a concrete example of a state of asubject block and adjacent blocks adjacent to the subject block. (a) ofFIG. 6 illustrates (i) a leftward adjacent block L whose estimatedprediction mode candidate mL is 1 (mode 1), (ii) an upward adjacentblock U whose estimated prediction mode candidate mU is 1 (mode 1), and(iii) an upper-leftward adjacent block UL whose estimated predictionmode candidate mUL is 8 (mode 8). The estimated prediction mode decidingsection 122 a decides with reference to the table of FIG. 5 that thecontribution degrees cL, cU, and cUL are 1.0, 0.0, and 0.1,respectively. The estimated prediction mode candidate mL of the leftwardadjacent block L is mode 1 (horizontal direction). This means that pixelvalues in the horizontal direction around the leftward adjacent block Lare likely to be remarkably strongly correlated with each other.Therefore, the contribution degree of the leftward adjacent block L isset high in mode 1. Meanwhile, though the estimated prediction modecandidate mU of the upward adjacent block U is mode 1, the contributiondegree of the upward adjacent block is set low in mode 1. This isbecause the upward adjacent block U is not horizontal to the subjectblock. In a case where the estimated prediction mode candidate mUL ofthe upper-leftward adjacent block UL is mode 8, pixel values do not haveany correlation in a particular direction. Therefore, the contributiondegree of the upper-leftward adjacent block UL is set low, that is, setto 0.1. The contribution degrees are set for respective combinations ofthe adjacent blocks and the modes in the table of FIG. 5, inconsideration of (i) a strong correlation direction and (ii) apositional relationship between the subject block and the adjacentblocks. It is therefore possible to increase how probability anestimated prediction mode becomes identical to an actual predictionmode, by employing how much pixel values are correlated with each other.

Step S4 a

The following describes step S4 a with reference to FIG. 4.

Step S3 a is a process which uses the table so that the estimatedprediction mode deriving section 1221 a derives the contribution degreeswith a small amount of processing at high speed. The contributiondegrees can be derived by other methods as long as a coder and a decoderemploy an identical method for deriving the contribution degrees.Examples of the other methods include (i) a method for deriving acontribution degree by calculating the contribution degree from an angleindicated by a prediction direction and (ii) a method for furtheradjusting, in accordance with characteristics and states of a subjectblock and an adjacent block adjacent to the subject block, acontribution degree derived with reference to a table. The method foradjusting a derived contribution degree is described as step S4 a asfollows.

(b) and (c) of FIG. 6 each are a view illustrating a case where asubject block C has a size different from that of a leftward adjacentblock L which is adjacent to a left side of the subject block C. (b) ofFIG. 6 is a view illustrating a case where the leftward adjacent block Lhas a side which is half as long as that of the subject block C. Acontribution degree cL of 1.0 can be derived from a prediction direction(mL=1: horizontal direction) of the leftward adjacent block Lillustrated in (b) of FIG. 6 with reference to the table of FIG. 5. Itis, however, considered that the adjacent block L has a low contributiondegree for a part of the subject block C which part is not in contactwith the leftward adjacent block L since the size of the subject block Cis different from that of the leftward adjacent block L. Further, suchdifference in block size causes difference in characteristic of regions.This means there is a probability that the leftward adjacent block L hasa low contribution degree. In order to prevent the contribution degreefrom becoming low, it is preferable to reduce the contribution degree onthe basis of a ratio of a surface area of the subject block C to that ofthe leftward adjacent block L. Note that the ratio is calculated suchthat a surface area of a smaller block is always set to a numerator soas to maximize the contribution degree in a case where the subject blockC has a size equal to that of the leftward adjacent block L. In a casewhere such reduction in contribution degree is applied to the case of(b) of FIG. 6, a coefficient is ¼, and the contribution degree cL is0.25 (=1.0.times.¼).

(c) of FIG. 6 is a view illustrating a case where the leftward adjacentblock L has a side which is twice as long as that of the subject blockC. In the case, it is also preferable to adjust the contribution degreeof the leftward adjacent block L in consideration of a probability thatthe contribution degree is not actually high, as with the case of (b) ofFIG. 6. In a case where such adjustment of the contribution degree isapplied to the case of (c) of FIG. 6, a coefficient is ¼, and thecontribution degree cL is 0.25 (=1.0.times.¼). By carrying out suchadjustment, the estimated prediction mode deriving section 1221 a canavoid overestimating the contribution degree in the case of thedifference in block size.

The method for adjusting a contribution degree is applicable to a casewhere a process unit does not have a square shape.

Step S5 a

Lastly, the estimated prediction mode deriving section 1221 a comparesthe contribution degrees cL, cU, and cUL of the respective estimatedprediction mode candidates mL, mU, and mUL with one another. Theestimated prediction mode deriving section 1221 a then derives Pestimated prediction mode(s) which is(are) in order of decreasing thecontribution degrees, the P being indicated by an estimating number. Forexample, in a case where the estimating number P is 2, the estimatedprediction mode deriving section 1221 a outputs a first estimatedprediction mode p1 and a second estimated prediction mode p2. In (a) ofFIG. 6, the contribution degree cL is 1.0, the contribution degree cU is0.0, and the contribution degree cUL is 0.1. In the case of (a) of FIG.6, the estimated prediction mode candidate mL (mode 1) and the estimatedprediction mode candidate mUL (mode 8) are outputted as the firstestimated prediction mode p1 and the second estimated prediction modep2, respectively.

The estimated prediction mode deriving section 1221 a outputs the Pestimated prediction mode(s) and the estimating number P as estimatedprediction mode information. Such ranking in order of the contributiondegrees makes it possible to derive a most probable prediction mode asthe first estimated prediction mode, and other probable prediction modesas second and latter estimated prediction modes.

A process to be carried out by the estimated prediction mode derivingsection 1221 a is not limited to the above-described explanation. Thefollowing exemplifies other methods carried out in the respective steps.

In step S1 a, how to decide the total number M of prediction modes, andhow much a value of the total number M of prediction modes is may not benecessarily decided as early described. The total number M of predictionmodes may be explicitly set for each subject block by, for example, (i)employing a flag for explicitly selecting the total number M ofprediction modes of 9 and the total number M of prediction modes of 17and (ii) deciding a value of the flag prior to a prediction mode for theeach subject block, (or by decoding in a case of a decoder). Suchsetting of the total number M of prediction modes makes it possible toadaptively set the total number M of prediction modes in accordance witha characteristic of an image without depending on a block size. Suchsetting of the total number M of prediction modes without depending onthe block size allows the estimated prediction mode deriving section1221 a to be configured not to use the block size.

Further, in step S1 a, how to obtain the estimating number P is notlimited to the above-described explanation. It is assumed in Embodiment1 that the total number of prediction modes is decided on the basis of ablock size. It is therefore possible to derive the estimating number P,for example, on the basis of a block size of a subject block.Specifically, in a case where the block size is 16.times.16 or32.times.32, the estimating number P is set to 2. In other cases, theestimating number P is set to 1. Alternatively, the estimating number Pcan be derived on the basis of a result obtained by comparing a blocksize of a subject block with a block size of a reference block.Specifically, in a case where a reference block includes a block whoseblock size is equal to that of a subject block, the estimating number Pis set to 1. This is because in the case, an estimated prediction modecandidate having a high contribution degree is present, wherebyprobability that an estimated prediction mode is identical to an actualprediction mode is increased. In other cases, the estimating number P isset to a relatively large number, that is, 2. This is because theestimated prediction mode is unlikely to be identical to the actualprediction mode.

The estimating number P is obtained for each block in step S1 a, asearly described. Alternatively, in step S1 a, the estimating number Pcan be set to an optimal value for a process unit larger than a block,such as a slice or a picture, though. In a case where the estimatingnumber P is obtained for the process unit larger than the block, anestimating number Pd of a default for the slice or the picture is setand coded, and the estimating number Pd is referred to in step S1 a.

In step S2 a, blocks to be selected are not limited to theabove-described three blocks. The three blocks are suitable ascomparison blocks in a case where a process in raster scan order from aleft upward direction to a right downward direction is assumed. This isbecause the three blocks have been already processed before the subjectblock is processed, that is, prediction modes of the three blocks havebeen decided. Instead of the three blocks, for example, other blockssuch as a block adjacent to an upper right side of the subject block ora block adjacent to a right side or a lower left side of the subjectblock can be selected as long as the other blocks have been alreadyprocessed.

Further, in step S2 a, for example, in a case where a plurality ofblocks are adjacent to a side of a subject block, the largest one of theplurality of blocks may be selected. Alternatively, in the case, all ofthe plurality of blocks may be selected. Further, the reference blockselecting section 1223 a may select a block(s) close to the subjectblock, such as a block adjacent to an adjacent block adjacent to thesubject block, a block included in a predetermined range from thesubject block, or all blocks which have been processed before thesubject block is processed. Such change in range of reference blocks tobe selected makes it possible to adjust flexibility to change in localimage characteristic.

Further, in step S2 a, the reference blocks to be selected can bechanged in accordance with the estimating number P supplied by theestimating number deciding section 1222 a. For example, only in a casewhere the estimating number P is 1, the leftward adjacent block and theupward adjacent block may be selected as the reference blocks.

An order of carrying out step S1 a and step S2 a can be reversedaccording to (i) details of step S1 a and step S2 a or (ii) mutualdependence of pieces of output data of the respective steps of S1 a andS2 a.

In step S3 a, the values in the table are not limited to an example ofFIG. 5. Note that it is preferable to set each of the values inconsideration of (i) a prediction direction of an adjacent blockadjacent to a subject block and (ii) a positional relationship betweenthe subject block and the adjacent block in a case where an estimatedprediction mode candidate is derived for each adjacent direction of ablock. Further, how to derive an estimated prediction mode candidate isnot limited to the above-described method. Particularly, in a case wherea plurality of reference blocks are selected, it is possible to reducean influence of change in local image characteristic during derivationof an estimated prediction mode, by employing, as the estimatedprediction mode, a prediction mode which has been statistically derivedfrom a group of the plurality of reference blocks. In step S2 a, forexample, in a case where all blocks adjacent to a left side of a subjectblock (leftward adjacent blocks), and all blocks adjacent to an upperside of the subject block (upward adjacent blocks) are selected asreference blocks, most ones of prediction modes of the respectiveleftward adjacent blocks, and most ones of prediction modes of therespective upward adjacent blocks may be derived as the estimatedprediction mode candidate. Instead of the most prediction modes, aprediction mode which is closest to an average prediction direction maybe derived as the estimated prediction mode candidate.

Further, in step S3 a, the table may be switched over in accordance withthe estimating number P. For example, in a case where the referenceblock selecting section 1223 a changes, in accordance with theestimating number P, how to select a reference block, the table isswitched over in accordance with a reference block to be selected.

In step S4 a, a contribution degree can be adjusted on the basis of ameasure other than a surface area ratio. For example, the contributiondegree can be adjusted on the basis of a ratio of a length of a side ofa subject block to a length of a side of an adjacent block adjacent tothe subject block, the sides being in contact with each other. It ispossible to obtain a contribution degree cL of 0.5 by, in step S4 a,adjusting the contribution degree cL at a ratio of ½ which is the ratioof the length of the side of the subject block to the length of the sideof the adjacent block. How to adjust a contribution degree on the basisof the ratio is useful for a case where a process unit is not a squareshape. Alternatively, the contribution degree may be adjusted (i)according to a distance between a subject block and a reference block or(ii) on the basis of the number of reference blocks which have been usedso as to derive one estimated prediction mode candidate.

Instead of adjusting the contribution degree by calculating it in stepS4 a, various determinations which are to be carried out in step S4 aare carried out prior to step S3 a, and the table to be used in step S3a, itself, may be changed on the basis of results of the variousdeterminations.

In step S5 a, the estimating number P may be changed on the basis of (i)a result obtained by deriving an estimated prediction mode candidate or(ii) a result obtained by deriving a contribution degree. For example,in a case where the estimating number P is 2 and three estimatedprediction mode candidates are identical to one another (ml), only onetype of estimated prediction mode is present. In the case, by employingthe ml as a first estimated prediction mode and changing the estimatingnumber P to 1, it is possible to avoid wastefully using a plurality ofidentical estimated prediction modes. As another example, in a casewhere (i) the estimating number P is 2, (ii) estimated prediction modecandidates mL, mU, and mUL are different from one another, and (iii)contribution degrees cL, cU, and cUL are equal to one another, it isdetermined that the estimated prediction mode candidates mL, mU, and mULhave an equal contribution degree. In the case, wastefully generating anestimated prediction mode candidate which is not to be used as anestimated prediction mode though the estimated prediction mode candidatehas the equal contribution degree can be avoided by (i) changing theestimating number P to 3 and (ii) setting the estimated prediction modecandidates mL, mU, and mUL to a first estimated prediction mode p1, asecond estimated prediction mode p2, and a third estimated predictionmode p3, respectively (the order of the estimated prediction modecandidates mL, mU, and mUL is not limited to this).

Further, in step S5 a, some or all of the estimated prediction modes maybe derived on the basis of something other than the estimated predictionmode candidates for example, on the basis of the estimating number P. Inthe case, the first estimated prediction mode through the P−1-thestimated prediction mode are derived on the basis of the resultobtained by deriving a contribution degree, as early described, and theP-th estimated prediction mode may be set to mode 8. This allows theestimated prediction modes to include a prediction mode which isgenerally in frequent use, without influence by local imagecharacteristic.

As further another example of step S5 a, in a case where the estimatingnumber P is 2, the first estimated prediction mode is derived on thebasis of the result obtained by deriving a contribution degree, and thesecond estimated prediction mode may be set to another prediction modehaving a value approximate to that of the first estimated predictionmode. Specifically, a prediction mode having the second largest modevalue following the largest mode value of the first estimated predictionmode may be set to the second estimated prediction mode. Alternatively,one of prediction modes each of which is correlated with a maindirection such as a horizontal direction, a vertical direction, or adirection of an angle of 45.degree., which one is closest to the firstestimated prediction mode, may be set to the second estimated predictionmode. It is therefore possible to increase how probable an estimatedprediction mode becomes identical to an actual prediction mode even in acase where the estimated prediction mode and the actual prediction modeare in the substantially same direction but have respective differentvalues, by setting, to the second estimated prediction mode, aprediction direction which is substantially identical but different fromthat of the first estimated prediction mode.

The process to be carried out by the estimated prediction mode derivingsection 1221 a may be changed in accordance with the estimating numberP. For example, in a case where the estimating number P is not less than2, steps S1 a through S5 s are carried out as they are. In a case wherethe estimating number P is 1, a simple method which is the same as thatof a conventional technique may be carried out, that is, one of aprediction mode for a leftward adjacent block and a prediction mode foran upward adjacent block, which one has a smaller value, may be set toan estimated prediction mode.

The following specifically describes another example of the process tobe carried out by the estimated prediction mode deriving section 1221 a.The estimated prediction mode deriving section 1221 a may obtain anestimated prediction mode by ranking and comparing prediction modes ofrespective adjacent blocks. For example, the prediction modes of therespective adjacent blocks are ranked in order of increasing values ofthe respective prediction modes or in order of decreasing probabilitiesof generation of the respective prediction modes. The ranking is carriedout with elimination of duplication of a prediction mode having a valueequal to that of another prediction mode in the prediction modes or aprediction mode having a probability of generation equal to that ofanother prediction mode in the prediction modes. P prediction mode(s),from the first prediction mode, of the prediction modes of therespective adjacent blocks may be employed as an estimated predictionmode(s). The order may be coded and transmitted by a coder (the movingimage coding device 1 a). Alternatively, the order may be derived frominformation decoded by a decoder (a moving image decoding device 2 a).Note, however, that such elimination of the duplication of theprediction mode causes a case where the number of estimated predictionmodes is less than P even in a case where all prediction modes of therespective adjacent blocks are employed as the estimated predictionmodes. This is because the number of prediction modes to be ranked isreduced by such elimination of the duplication of the prediction mode.In order to deal with the case, a predetermined prediction mode whichhas not been employed as the estimated prediction modes may be added sothat the number of estimated prediction modes becomes P. Thepredetermined prediction mode to be added can be determined so that thecoder and the decoder employ an identical prediction mode. Examples ofthe predetermined prediction mode include (i) a DC mode, (ii) a Planarmode, (iii) a horizontal or vertical direction prediction mode, (iv) aprediction mode whose direction is closest to an intermediate directionbetween a direction indicated by a prediction mode for a leftwardadjacent block and a direction indicated by a prediction mode for anupward adjacent block, and (v) a prediction mode whose direction isclose to that of a prediction mode which is employed as an estimatedprediction mode. Note also that the duplication of the prediction modesis not necessarily duplication of prediction modes which are completelyidentical to another prediction mode but can be duplication ofprediction modes whose directions are not greatly different from eachother or prediction modes obtained by the DC mode with the Planar mode,respectively.

The following describes the prediction mode coding section 123 a withreference to FIG. 1. The prediction mode coding section 123 a codes aprediction mode on the basis of a block size and estimated predictionmode information to obtain prediction information coded data, and thenoutputs the prediction information coded data. This is specificallyexplained with reference to FIG. 7.

FIG. 7 is a table showing a concrete example of how the prediction modecoding section 123 a codes a prediction mode in a case where anestimating number P is 2. The estimating number P is supplied to theprediction mode coding section 123 a as a piece of the estimatedprediction mode information. In FIG. 7, p.sub.min represents either oneof a first estimated prediction mode p1 and a second estimatedprediction mode p2 which has a smaller value, and p.sub.max representseither one of the first estimated prediction mode p1 and the secondestimated prediction mode p2 which has a larger value. Other referencenumerals which have been already described are not described here.

The prediction mode coding section 123 a outputs a flag_bit0, andfurther outputs a flag_bit1 and a rem_bits according to conditions metby a prediction mode m (see FIG. 7). The conditions, and operationscarried out by the prediction mode coding section 123 a under therespective conditions are described as follows.

(a) In a case where the prediction mode m is identical to the firstestimated prediction mode p1, the prediction mode coding section 123 aoutputs merely a one-bit flag_bit0=1 as coded data.

(b) In a case where the prediction mode m is not identical to the firstestimated prediction mode p1 but is identical to the second estimatedprediction mode p2, the prediction mode coding section 123 a outputs aflag_bit0=0 and a two-bit flag_bit1=1.

(c) In a case where the prediction mode m is neither identical to thefirst estimated prediction mode p1 nor identical to the second estimatedprediction mode p2, the prediction mode coding section 123 a outputs aflag_bit0=0 and a flag_bit1=0, and further outputs a bit string rem_bitsderived by any one of the following methods in accordance with the valueof the prediction mode m.

(c1) In a case where m<p.sub.min, the prediction mode coding section 123a converts the prediction mode m into a bit string rem_bits.

(c2) In a case where p.sub.min<m<p.sub.max, the prediction mode codingsection 123 a converts a prediction mode m-1 into a bit string rem_bits.

(c3) In a case where p.sub.max<m, the prediction mode coding section 123a converts a prediction mode m-2 into a bit string rem_bits.

A process carried out by the prediction mode coding section 123 a underthe condition (c) is useful for improvement of a coding efficiency byeliminating a plurality of estimated prediction modes from a range ofvalues of the rem_bits so as to reduce the range, as with a processcarried out by the conventional prediction mode coding section 123.

The following describes another concrete example of how the predictionmode coding section 123 a codes a prediction mode. In the case where theestimating number P is 2, the prediction mode coding section 123 aoutputs (i) a flag flag_bitG indicating that a prediction mode isidentical to any one of estimated prediction modes which is included ina group G of the estimated prediction modes and (ii) an identifiermpm_idx on the basis of which one of the estimated prediction modes isselected. Conditions and operations carried out by the prediction modecoding section 123 a under the respective conditions are described asfollows.

(d) In a case where the prediction mode m is identical to any one of theestimated prediction modes which is included in the group G, that is,the first estimated prediction mode p1 or the second estimatedprediction mode p2, the prediction mode coding section 123 a outputs aflag_bitG=1 as coded data. The prediction mode coding section 123 afurther outputs the identifier mpm_idx indicative of an estimatedprediction mode identical to the prediction mode m. In the case wherethe estimating number P is 2, the identifier mpm_idx is a one-bitidentifier. Note that in a case where the first estimated predictionmode p1 is identical to the second estimated prediction mode p2, thevalue of the prediction mode m is obvious. It is therefore possible toimprove the coding efficiency by not coding the identifier mpm_idx.

(e) In a case where the prediction mode m is neither identical to thefirst estimated prediction mode p1 nor identical to the second estimatedprediction mode p2, the prediction mode coding section 123 a outputs aflag_bitG=0 as coded data. The prediction mode coding section 123 afurther derives and outputs a rem_bits by any one of the methods (c1)through (c3) in accordance with the value of the prediction mode m.

Thus, the prediction mode coding section 123 a generates coded data froma prediction mode on the basis of a plurality of estimated predictionmodes. In a case where a subject block actually has any one of theplurality of estimated prediction modes, the prediction mode codingsection 123 a can code the prediction mode with less code amount.Further, employment of the plurality of estimated prediction modes makesit possible to increase how probable an estimated prediction mode isidentical to an actual prediction mode even in a block having a largenumber of prediction modes. Embodiment 1 described the case where theestimating number P is 2. In a case where the estimating number P is 1,the prediction mode coding section 123 a carries out coding identical tothat carried out by the prediction mode coding section 123. Even in acase where the estimating number P is not less than 3, the predictionmode coding section 123 a can code a prediction mode by carrying out, byuse of three or more-bit flags that are similar to the respectiveflag_bit0 and flag_bit1, coding identical to that carried out in thecase where the estimating number P is 2.

That is, even in the case where the estimating number P is not less than3, the prediction mode coding section 123 a can code a prediction modeas with the case where the estimating number is 2 (i) by use of theflags that are similar to the respective flag_bit0 and flag_bit1 andfurther (ii) by eliminating the estimated prediction modes from therange of the values of the rem_bits so as to reduce the range as withthe condition (c). Alternatively, even in the case where the estimatingnumber P is not less than 3, the prediction mode coding section 123 acan code a prediction mode by carrying out the method described in theanother concrete example by use of the flag flag_bitG and the identifiermpm_idx which has a range of values in accordance with the estimatingnumber P.

Thus, it is possible to reduce whole code amount of prediction modes incoded data to be outputted from the moving image coding device 1 a by(i) coding in advance information indicative of whether or not an actualprediction mode is identical to any one of a plurality of estimatedprediction modes and (ii) coding the actual prediction mode only in acase where the actual prediction mode is not identical to any one of theplurality of estimated prediction modes.

Thus, the moving image coding device 1 a derives a plurality of rankedestimated prediction modes which are more accurate than a conventionaltechnique, on the basis of contribution degrees of respective estimatedprediction mode candidates as which prediction modes of respectiveadjacent blocks adjacent to a subject block are employed. Further,employment of the plurality of ranked estimated prediction modes forgeneration of coded data from the prediction modes makes it possible toincrease a probability of coding with less code amount by use of asecond estimated prediction mode even in a case where a first estimatedprediction mode is not identical to a prediction mode for the subjectblock, thereby improving the coding efficiency.

Moving Image Decoding Device

The following explains a moving image decoding device 2 a according toan embodiment of the present invention, with reference to FIGS. 8through 10. In brief, the moving image decoding device 2 a of the firstembodiment of the present invention is a device that, like aconventional moving image decoding device 2, decodes coded data of eachmacro block unit and outputs a decoded image. The moving image decodingdevice 2 a is a moving image decoding device part of which utilizes atechnique adopted in H.264/AVC. FIG. 8 is a diagram illustrating aconfiguration of an MB decoding section 20 a in the moving imagedecoding device 2 a. The MB decoding section 20 a illustrated in FIG. 8is obtained by replacing, with a prediction information decoding section23 a, the prediction information decoding section 23 in the MB decodingsection 20 as illustrated in FIG. 13. The prediction informationdecoding section 23 a decodes prediction information decoded data andoutputs a prediction mode and a block size. In this point, theprediction information decoding section 23 a is similar to theprediction information decoding section 23 of the moving image decodingdevice 2. The following specifically explains a prediction mode decodingmethod which is different from that of the prediction informationdecoding section 23.

FIG. 9 is a diagram illustrating a configuration of the predictioninformation decoding section 23 a. The prediction information decodingsection 23 a includes a prediction mode decoding section 231 a, and theprediction mode recoding section 121 and the estimated prediction modedeciding section 122 a which are described above.

The following explains the prediction mode decoding section 231 a. Theprediction mode decoding section 231 a first decodes a block size fromprediction information coded data. Then, the prediction mode decodingsection 231 a decodes a prediction mode from the prediction informationcoded data in accordance with estimated prediction mode information thathas been derived by the estimated prediction mode deciding section 122 ausing thus decoded block size. This is explained more specifically, withreference to FIG. 10.

FIG. 10 is a table showing a specific example of a prediction modedecoding method in the prediction mode decoding section 231 a in a casewhere the estimating number P=2. Explanations of reference signsmentioned above are omitted here.

The prediction mode decoding section 231 a first decodes flag_bit0. Asshown in FIG. 10, when flag_bit0=1, an estimated prediction mode p1 isequal to the prediction mode m and it is unnecessary to decode flag_bit1and rem_bits. When flag_bit0=0, the prediction mode decoding section 231a then decodes flag_bit1. When flag_bit1=1, an estimated prediction modep2 is equal to a prediction mode m and it is unnecessary to decoderem_bits. When flag_bit0=0 and flag_bit1=0, no estimated prediction modeis identical to the prediction mode. In this case, the prediction modedecoding section 231 a converts subsequent B.sub.M′-bit coded data k asrem_bits into a decimal number m′, and derives the prediction mode m inaccordance with magnitude relations between m′ and p.sub.min and betweenm′ and p.sub.max.

As described above, the prediction mode decoding section 231 a decodes aprediction mode from coded data, in accordance with a plurality ofestimated prediction modes. Here, a case where the estimating number P=2is explained. When P=1, the prediction mode decoding section 231 ashould perform decoding in the same manner as the prediction modedecoding section 231. When P.gtoreq.3, decoding should be carried out,for example, in the same manner as in a case where P=2. In this casewhere P.gtoreq.3, decoding of a prediction mode is possible byproviding, in coded data, a flag having three or more bits that aresimilar to flag_bit0 and flag_bit1.

The same effect as described above can be obtained, even when thefollowing configuration other than a configuration including flag_bit0and flag_bit1 is alternatively employed as a configuration of a flagthat indicates whether or not a prediction mode and each estimatedprediction mode are identical. In this alternative configuration, forexample, when the estimating number P=2, the prediction mode decodingsection 231 a first decodes a flag, that is, flag_bitG. This flag_bitGindicates that an actual prediction mode is identical to one estimatedprediction mode in a group G including two estimated prediction modes.flag_bitG=0 indicates that neither one of the two estimated predictionmodes of the group G is identical to the actual prediction mode. Thus, aprediction mode is decided by decoding rem_bits. Meanwhile, flag_bitG=1indicates that one of the two estimated prediction modes of the group Gis identical to the actual prediction mode. In this case, the predictionmode decoding section 231 a further decodes an identifier mpm_idx thatindicates which one of the two estimated prediction modes of the group Gis identical to the actual prediction mode. When the identifiermpm_idx=0, the estimated prediction mode p1 is identical to the actualprediction mode. Meanwhile, when mpm_idx=1, the estimated predictionmode p2 is identical to the actual prediction mode. Similarly, even whenthe estimating number.gtoreq.3, which estimated prediction mode isidentical to an actual prediction mode can be indicated by using anidentifier mpm_idx in a case where flag_bitG=1. Note that a code lengthof the identifier mpm_idx is desirably shorter than B.sub.M′ digits interms of a bit stream.

Further, in the above configuration, estimated prediction modes aredefined as one group G. However, a plurality of estimated predictionmodes can be divided into two or more groups. In this case, for each oneof the two or more groups, it is possible to decode a flag indicatingthat an actual prediction mode is identical to any one of the estimatedprediction modes constituting one of the two or more groups. Further, ina case where an actual prediction mode is identical to any one of theestimated prediction modes constituting the one of the two or moregroups as a result of determination of a flag, it is also possible tofurther decode an index that indicates an estimated prediction mode thatis identical to the actual prediction mode which estimated predictionmode is from among the estimated prediction modes constituting the onegroup. The flag and index are the same as the flag_bitG and mpm_idxabove.

As described above, in coded data that is to be inputted into the movingimage decoding device 2 a, first decoded is information on identicalnessof a plurality of estimated prediction modes and an actual predictionmode. Only when the actual prediction mode is not identical with any ofthe plurality of estimated prediction modes, information for designatinga prediction mode is further decoded. This makes it possible to reduce atotal amount of codes for prediction modes. Note that the estimatedprediction modes are preferably calculated from a prediction mode for anadjacent block. Further, the number of estimated prediction modes ispreferably changed in accordance with the number of prediction modes tobe used. For example, the number of prediction modes can be set to 2 ina case where a predetermined constant M is used and the number ofprediction modes is not less than M; meanwhile, in a case where thenumber of prediction modes is less than M, the number of estimatedprediction mode can be set to 1. As in this example, when the number ofprediction modes is larger, the number of estimated prediction modes ispreferably set larger. Alternatively, for example, in a case where apredetermined constant M is used and the number of prediction modes isnot less than M, the number of estimated prediction modes is preferablyset to 3; meanwhile, in a case where the number of prediction modes isless than M, the number of estimated prediction modes is preferably setto 2.

It is also preferable to alternatively employ a configuration where, forexample, the number of estimated prediction modes are decided so thatthe number of remaining prediction modes is 2.sup.n. When the number ofremaining prediction modes is 2.sup.n, coding can be performed at asubstantially equal probability by using a fixed bit number. In thissense, for example, in a case where the number of prediction modes is18, the number of estimated prediction modes is preferably set to 2.This is because the number of remaining prediction modes is 16 in thiscase. Further, for example, in a case where the number of predictionmodes is 35, the number of estimated prediction modes is preferably setto 3. This is because the number of remaining prediction modes is 32 inthis case.

As described above, by deriving a plurality of estimated predictionmodes ordered according to a method that is employed in the moving imagedecoding device 1 a, the moving image decoding device 2 a can decodecoded data that is generated by the moving image coding device 1 a.

Additional Matter 1

The present embodiment has discussed a method of coding/decoding a flagwhich indicates that an actual prediction mode is identical with each ofestimated prediction modes. Note however that the other methods can beused. For example, a method can be such that a flag indicating that anactual prediction mode is identical with any one of a plurality ofestimated prediction modes is coded, and, in addition, in a case wherethe actual prediction mode is identical with the any one of theplurality of estimated prediction modes, identification data to beassociated with the any one of the plurality of estimated predictionmodes can be included in coded data. In this case, it is preferable thata length of a code of the identification data be shorter in bitstreamthan B.sub.M figure.

Additional Matter 2

According to the present embodiment, determination is carried out ineach step with the use of a block size. Note however that thedetermination can be carried out without the use of the block size. Anexample is a case where a total number of prediction modes for each ofsubject blocks does not depend on its block size and is thus constant.

Additional Matter 3

The present invention is not limited to the description of theembodiments above, but may be altered by a skilled person within thescope of the claims. An embodiment based on a proper combination oftechnical means disclosed in different embodiments is encompassed in thetechnical scope of the present invention.

Lastly, each block of the aforementioned moving image decoding devices 1and 1 a and each block of the aforementioned moving image coding devices2 and 2 a may be realized by way of hardware as executed by a logiccircuit provided on an integrated circuit (IC chip), or by way ofsoftware as executed by a CPU (Central Processing Unit).

In a case of realizing by way of software, each of the devices includes(i) a CPU which executes a command of a program that implementsfunctions of the each of the moving image coding devices 1 and 1 a andthe each of the moving image decoding devices 2 and 2 a, (ii) a ROM(Read Only Memory) in which the program is stored, (iii) a RAM (RandomAccess Memory) which derives the program, and (iv) a storage device (arecording medium) such as a memory in which the program and various setsof data are stored. The object of the present invention can be alsoattained as follows. That is, a recording medium in which program codes(an executable program, an intermediate code program, and a sourceprogram) of a control program of the each of the devices, which aresoftware that implement the functions, are computer-readably recorded issupplied to the each of the devices, and a computer of the each of thedevices (a CPU or an MPU) reads and carries out the program codesrecorded in the storage medium.

Examples of the recording medium include (i) tapes such as a magnetictape and a cassette tape, (ii) disks including magnetic disks such as aFloppy (Registered Trademark) disk and a hard disk, and optical diskssuch as a CD-ROM, an MO, an MD, a DVD, a CD-R, and a Blu-ray Disc(Registered Trademark), (iii) cards such as an IC card (including amemory card) and an optical card, (iv) semiconductor memories realizedby a mask ROM, EPROM, EEPROM, a flash ROM, and the like, and (v) logiccircuits such as PLD (Programmable logic device) and FPGA (FieldProgrammable Gate Array).

Further, the each of the devices can be arranged to be connectable to acommunication network so that the program codes can be supplied to theeach of the devices via the communications network. The communicationnetwork is not limited in particular, provided that the communicationnetwork is capable of supplying the program codes to the each of thedevices. Examples of the communication network includes the Internet, anintranet, an extranet, a LAN, ISDN, VAN, a CATV communications network,a virtual private network, a telephone network, a mobiletelecommunications network, and a satellite communication network. Atransmission medium of which a communication network is composed is notlimited in particular, provided that the transmission medium is capableof transmitting the program codes. Examples of the transmission mediumincludes wired transmission media such as IEEE1394, a USB, a power-linecarrier, a cable TV circuit, a telephone line, and ADSL (AsymmetricDigital Subscriber Line) and wireless transmission media such asinfrared communication systems such as IrDA and a remote controller,Bluetooth (Registered Trademark), IEEE802.11 wireless communicationsystem, HDR (High Data Rate), NFC (Near Filed Communication), DLNA(Digital Living Network Alliance), a mobile phone network, a satellitecircuit, and a digital terrestrial network.

Additional Matter 4

The present invention can be described as below. An intra-predictionmode estimation device according to one aspect of the present inventionis an intra-prediction mode estimation device for performingintra-prediction mode estimation of an image, in a coding process or adecoding process for a digital image constituted of a plurality ofblocks, comprising: a selecting section for selecting one or morereference blocks for comparison with a subject block, which is a subjectfor the intra-prediction mode estimation; an estimating number decidingsection for deciding a total number of estimated prediction modes to beestimated as estimated values in the intra-prediction mode estimationfor the subject block; and an estimating section for performing theintra-prediction mode estimation for the total number of the estimatedprediction modes with respect to the subject block, based on aprediction mode used for a coding process of the reference block thusselected.

A coding device according to another aspect of the present invention isa coding device for coding a digital image constituted by a plurality ofblocks, comprising: a selecting section for selecting one or morereference blocks for a subject block, which is a subject for the coding;an estimating number deciding section for deciding a total number ofestimated prediction modes to be estimated as estimated values inintra-prediction mode estimation for the subject block; an estimatingsection for performing the intra-prediction mode estimation for thetotal number of the estimated prediction modes with respect to thesubject block, based on a prediction mode used for a coding process ofthe reference block thus selected; a prediction mode deciding sectionfor deciding a prediction mode for the subject block; a prediction modecoding section for coding the estimated prediction modes and theprediction mode in such a way that a code length of the coding isdifferent according to whether or not the prediction mode is identicalwith any one of the estimated prediction modes; and a predicted imagegenerating section for generating a predicted image of the subjectblock, based on the prediction mode, the prediction mode coding sectionperforming the coding in such a way that the code length of the codingis shorter when the prediction mode is identical with any one of theestimated prediction modes than when the prediction mode is notidentical with any one of the estimated prediction modes.

A decoding device according to still another aspect of the presentinvention is a decoding device for decoding a coded data of a digitalimage constituted by a plurality of blocks, comprising: a selectingsection for selecting one or more reference blocks for a subject block,which is a subject for the decoding; an estimating number decidingsection for deciding a total number of estimated prediction modes to beestimated as estimated values in intra-prediction mode estimation forthe subject block; an estimating section for performing theintra-prediction mode estimation for the total number of the estimatedprediction modes with respect to the subject block, based on aprediction mode used for a coding process of the reference block thusselected; a prediction mode decoding section for decoding, from thecoded data, a prediction mode to be used in generation of predictedimage of the subject block, the prediction mode decoding sectionperforming the decoding based on the total number of the estimatedprediction modes and the estimated prediction modes; and a predictedimage generating section for generating a predicted image of the subjectblock, based on the prediction mode.

Coded data according to yet another aspect of the present invention iscoded data of an image, which coded data, constituted by a plurality ofblocks, is outputted or is to be outputted from the above-mentionedcoding device, the coded data comprising respective pieces ofinformation regarding each of the plurality of blocks, the respectivepieces of information being individually: a) information indicating thata first estimated prediction mode for the block is identical with aprediction mode for the block; b) information indicating that a secondestimated prediction mode but not the first estimated prediction mode isidentical with the prediction mode; or c) (i) information indicatingthat the first estimated prediction mode and the second estimatedprediction mode are not identical with the prediction mode, and (ii)information regarding the prediction mode.

Coded data according to still yet another aspect of the presentinvention is coded data of an image, which coded data, constituted by aplurality of blocks, is outputted or is to be outputted from theabove-mentioned coding device, the coded data comprising respectivepieces of information regarding each of the plurality of blocks, therespective pieces of information being individually: a) (i) informationindicating that an estimated prediction mode included in the groupconsisting of one or more estimated prediction modes for the block isidentical with the prediction mode for the block, and (ii) informationdesignating that estimated prediction mode; or b) (i) informationindicating that no estimated prediction mode included in the groupconsisting of one or more estimated prediction modes for the block isidentical with the prediction mode for the block, and (ii) informationregarding the prediction mode.

According to the above configuration, it is possible to adaptivelydecide, for each of subject blocks, a total number of estimatedprediction modes to be estimated. Generally, the use of a large numberof estimated prediction modes increases a possibility that one of thelarge number of estimated prediction modes is identical with an actualprediction mode. However, in a case where none of the large number ofestimated prediction modes is identical with the actual prediction mode,this causes an increase in an amount of coded data. Therefore, accordingto the above configuration, it is possible to yield an effect ofefficiently coding/decoding prediction modes with a smaller amount ofcoded data than average by adaptively deciding a total number ofestimated prediction modes to be estimated.

The intra-prediction mode estimation device according to the presentinvention is preferably configured such that the estimating numberdeciding section decides the total number of prediction modes accordingto a total number of the prediction modes used for a coding process ofthe subject block.

According to the above configuration, since a total number of estimatedprediction modes to be estimated is controlled according to a totalnumber of prediction modes used for a coding process of a subject block,it is possible to derive the number of the estimated prediction modeswhich number corresponds to the total number of the prediction modes,that is, a variety of choices of the prediction modes.

For example, in a case where a total number of prediction modes is high,it is unlikely that any of estimated prediction modes is identical withan actual prediction mode. Therefore, by increasing a total number ofthe estimated prediction modes to be estimated, it is possible toincrease a possibility that any of the estimated prediction modes isidentical with the actual prediction mode.

The intra-prediction mode estimation device according to the presentinvention is preferably configured such that the estimating numberdeciding section decides the total number of prediction modes such thata total number of prediction modes to be estimated for a first subjectblock is greater than that for a second subject block, if a total numberof prediction modes for a coding process of the first subject block isgreater than that of the second subject block.

According to the above configuration, the estimating number decidingsection decides the total number of the estimated prediction modes insuch a way that, for a subject block for which a total number ofprediction modes is large, the total number of the estimated predictionmodes to be estimated is large, and for a subject block for which atotal number of prediction modes is small, the total number of theestimated prediction modes to be estimated is small. This makes itpossible to prevent decrease in efficiency of coding which decrease iscaused due to use of a small number of estimated prediction modes for asubject block for which a total number of prediction modes is large.

The intra-prediction mode estimation device according to the presentinvention is preferably configured such that the estimating numberdeciding section decides the total number of prediction modes accordingto a dimension of the subject block.

According to the above configuration, the estimating number decidingsection decides, according to a dimension of a subject block, a totalnumber of estimated prediction modes to be estimated. Therefore, in acase where a total number of prediction modes is to be decided accordingto a dimension of a subject block, a total number of estimatedprediction modes to be estimated can be decided without reference to thetotal number of the prediction modes.

The intra-prediction mode estimation device according to the presentinvention is preferably configured such that the estimating numberdeciding section decides the total number of prediction modes accordingto dimensional relative relationship between the subjection block andthe reference block.

According to the above configuration, it is possible to decide that atotal number of estimated prediction modes to be estimated is a numbercorresponding to similarity of a subject block and a reference block.For example, difference in dimension between a subject block and areference block indicates that an image in the subject block may bedifferent in characteristics from an image in the reference block.Because any of estimated prediction modes which are derived with the useof the reference block having different characteristics is unlikelyidentical with an actual prediction mode, the use of a large value in atotal number of the estimated prediction modes to be estimated makes itpossible to increase a possibility that any of the estimated predictionmodes is identical with the actual prediction mode. Alternatively, in acase where the subject block is too different from the reference block,the use of a low value in the total number of the estimated predictionmodes to be estimated makes it possible to prevent increase in an amountof coded data.

The intra-prediction mode estimation device according to the presentinvention is preferably configured such that the estimating numberdeciding section determines the dimensional relative relationshipaccording to areas or side lengths.

According to the above configuration, it is possible to easily decidedimensional relative relationship according to areas or side lengths ofa subject block and a reference block. This makes it possible to preventincrease in an amount of calculation for evaluating the dimensionalrelative relationship.

The intra-prediction mode estimation device according to the presentinvention is preferably configured such that the estimating sectionfurther performs the prediction mode estimation based on positionalrelationship between the subject block and the reference block.

According to the above configuration, by performing prediction modeestimation based on positional relationship, it is possible to increasea possibility that any of estimated prediction modes is identical withan actual prediction mode. For example, it is determined that predictionmodes are more accurate as estimated prediction modes because, since aprediction direction indicated by each of the prediction modes indicatesa direction in which correlation between pixel values is strong,correlation between the prediction modes becomes high in a case where adirection indicating positional relationship between a subject block anda reference block is similar to the prediction direction for thereference block. This makes it possible to increase estimation accuracyof estimated prediction modes.

The intra-prediction mode estimation device according to the presentinvention is preferably configured such that the selecting sectionselects, as the reference block, a block for which a prediction mode hadbeen decided before the subject block.

According to the above configuration, a reference block holds, asinformation, prediction modes which have been actually used, and thus itis possible to estimate estimated prediction modes with the use of theinformation.

The intra-prediction mode estimation device according to the presentinvention is preferably configured such that the selecting sectionselects, as the reference block, blocks adjacent to the subject block ina left upward direction, an upward direction, and a leftward direction.

According to the above configuration, it is possible to estimateestimated prediction modes with the use of adjacent reference blocks.For example, in a case where a process is performed on blocks in rasterscan order, blocks adjacent to a subject block in an upward direction, aleftward direction, and a left upward direction have been processed, andthus prediction modes for the block have been decided. Therefore, theprocessed blocks are suitable for reference blocks. By performingestimation with the use of such adjacent blocks, it is possible toestimate estimated prediction modes while corresponding to localcharacteristics of an image.

Additional Matter 5

Each of the macro blocks of the embodiments corresponds to an LCU(Largest Coding Unit, the LCU is also referred to as a root of a CodingTree) of HEVC (High Efficiency Video Coding) which is proposed as asuccessive standard of H.264/MPEG-4 AVC. Each of the macro blocks andeach of the blocks corresponds to a CU (Coding Unit, the CU is alsoreferred to as a leaf of the Coding Tree), a PU (Prediction Unit), and aTU (Transformation Unit) of HEVC. The LCU is also referred to as a treeblock.

Additional Matter

The embodiments of the present invention have been specificallydiscussed with reference to the drawings. Note however that specificconfigurations are not limited to the embodiments, and design and thelike which do not exceed the content of the present invention are alsoencompassed in the patent claims.

INDUSTRIAL APPLICABILITY

The present invention can be preferably employed for a moving imageprocessing system including (i) a moving image coding device for codinga moving image and (ii) a moving image decoding device for decoding acoded image.

REFERENCE SIGNS LIST

-   -   1 Moving image coding device    -   1 a Moving image coding device (intra-prediction mode estimation        device, coding device)    -   2 Moving image decoding device    -   2 a Moving image decoding device (intra-prediction mode        estimation device, decoding device)    -   10 MB coding section    -   11 Prediction information deciding section    -   12 Prediction information coding section    -   13 Predicted image generating section    -   14 Prediction residual coding section    -   15 MB coded data multiplexing section    -   20 MB decoding section    -   21 MB coded data inverse multiplexing section    -   22 Prediction residual decoding section    -   23 Prediction information decoding section    -   121 Prediction mode recoding section    -   122 Estimated prediction mode deciding section    -   123 Prediction mode coding section    -   231 Prediction mode decoding section    -   1221 Estimated prediction mode deriving section    -   10 a MC coding section    -   12 a Prediction information coding section    -   20 a MB decoding section    -   23 a Prediction information decoding section    -   122 a Estimated prediction mode deciding section    -   123 a Prediction mode coding section    -   231 a Prediction mode decoding section    -   1221 a Estimated prediction mode deriving section (estimating        section)    -   1222 a Estimating number deciding section    -   1223 a Reference block selecting section (selecting section)

What is claimed is:
 1. A decoding device for decoding anintra-prediction mode of a digital image constituted by a plurality ofblocks, comprising: a memory; and a processor, wherein the processor isconfigured to: select one or more adjacent blocks for a subject block,which is a subject for the decoding; in a case where there areduplicated prediction modes in the adjacent blocks, add anintra-prediction mode whose direction is close to that of a predictionmode in the adjacent blocks to a group consisting of estimatedprediction modes for the subject block and further add predeterminedprediction modes to the group until the number of estimated predictionmodes reaches a predetermined number; and decode, for each of theplurality of blocks, a flag indicating whether or not an estimatedprediction mode included in the group is identical to the predictionmode for the subject block, in a case where the flag is 1, derive theprediction mode of the subject block by (a) decoding an identifiermpm_idx and (b) referring to a prediction mode, identified by mpm_idx,from among the estimated prediction modes included in the group, and ina case where the flag is 0, derive the prediction mode of the subjectblock by (a′) decoding rem_bits and (b′) referring to the rem_bits fromamong prediction modes which are not included in the group.
 2. Thedecoding device of claim 1 wherein the processor is further configuredto generate a predicted image of the subject block, based on the derivedprediction mode.
 3. A decoding method for decoding an intra-predictionmode of a digital image constituted by a plurality of blocks, the methodcomprising: selecting one or more adjacent blocks for a subject block,which is a subject for the decoding; adding, in a case where there areduplicated prediction modes in the adjacent blocks, an intra-predictionmode whose direction is close to that of a prediction mode in theadjacent blocks to a group consisting of estimated prediction modes forthe subject block and further adding predetermined prediction modes tothe group until the number of estimated prediction modes reaches apredetermined number; decoding, for each of the plurality of blocks, aflag indicating whether or not an estimated prediction mode included inthe group is identical to the prediction mode for the subject block, ina case where the flag is 1, the method further comprising deriving theprediction mode of the subject block by (a) decoding an identifiermpm_idx and (b) referring to a prediction mode, identified by mpm_idx,from among the estimated prediction modes included in the group, and ina case where the flag is 0, the method further comprising deriving theprediction mode of the subject block by (a′) decoding rem_bits and (b′)referring to the rem_bits from among prediction modes which are notincluded in the group.
 4. The decoding method of claim 3 furthercomprising generating a predicted image of the subject block, based onthe derived prediction mode.
 5. A non-transitory computer-readablestorage medium storing a program for causing a computer to decode anintra-prediction mode of a digital image constituted by a plurality ofblocks, wherein the program causes the computer to: select one or moreadjacent blocks for a subject block, which is a subject for thedecoding; in a case where there are duplicated prediction modes in theadjacent blocks, add an intra-prediction mode whose direction is closeto that of a prediction mode in the adjacent blocks to a groupconsisting of estimated prediction modes for the subject block andfurther adding predetermined prediction modes to the group until thenumber of estimated prediction modes reaches a predetermined number;decode, for each of the plurality of blocks, a flag indicating whetheror not an estimated prediction mode included in the group is identicalto the prediction mode for the subject block, in a case where the flagis 1, derive the prediction mode of the subject block by (a) decoding anidentifier mpm_idx and (b) referring to a prediction mode, identified bympm_idx, from among the estimated prediction modes included in thegroup, and in a case where the flag is 0, derive the prediction mode ofthe subject block by (a′) decoding rem_bits and (b′) referring to therem_bits from among prediction modes which are not included in thegroup.
 6. The non-transitory computer-readable storage medium of claim 5wherein the program further causes the computer to generate a predictedimage of the subject block, based on the derived prediction mode.